Electrical contacts



United States Patent O ELECTRICAL CONTACTS Frederick R. Faruham, Pittsburgh, Pa., assignor to Gibson Electric Company, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application September 24, 1952, Serial No. 311,331

2 Claims. (Cl. 29-195) My invention relates to electrical contacts used for circuit interrupting equipment and is more particularly related to (1) increasing the weldability of such contacts, and (2) making the respective surfaces of such contacts, with increased weldability characteristics, readily distinguishable. 7

An object of my invention is to provide a contact for circuit interrupting equipment whose back will make point contact with a contact arm to decrease thereby the effective cross-sectional area through which the welding current will pass and thereby concentrate and increase the welding heat. p

A further object of my invention is to provide an electrical contact with silver solder attached to the welding surface thereof to increase thereby the weldability of the contact.

Another object of my invention is to provide a contact with increased weldability characteristics so that the respective sides thereof are readily distinguishable thus enabling the operator to determine easily which surface is to engage the contact arm.

A still further object of my invention is to provide an electrical contact with both increased weldability properties and which has its respective surfaces readily distinguishable.

For the purposes of simplicity, my invention will be described in connection with contacts of a refractory type, such as silver-tungsten, which are produced by powder metallurgy means.

One method of making such electrical contacts is by mixing silver and a refractory metal powder such as molybdenum, tungsten or their carbides and pressing said mixture to form a porous compact, with or without a serrated back. This compressed body is then sintered at temperatures and for periods sufiiciently long to permit the particles to sinter together and form the porous refractory compact.

A high conductivity metal, such as silver or copper, is then impregnated into this porous composition to fill up the pores of the compact.

Ordinarily, this type of compact is caused to make, carry, and/or break rather large currents. In order to avoid increased resistance which results in overheating of the contacts and other parts of the device during normal operations, it is necessary to secure these contacts firmly to their respective contact arms.

Numerous means are in present use to accomplish this welding operation. .One such method is by brazing such sintered contacts to their supports with a brazing medium such as silver solder. The type of equipment most commonly used to do this is resistance brazing equipment or induction heating equipment. For the former, the parts are assembled in proper position and then placed between high resistance, as for instance graphite, electrodes and large currents are passed through the combination. This will heat up the electrodes and the assembly so as to cause the silver solder to alloy with, and thereby combine, the components of the assembly. Pressure is usually applied simultaneousy with the heat. This has the decided advantages of forcing entrapped air and flux out of the joint as well as providing for a minimum thickness of solder in the joint.

When the contacts are attached to circuit breaker contact arms by means of induction heating, induction coils are placed around the combination and heat is furnished by means of high frequency passing through these coils. One disadvantage of this process is that the desirable application of pressure is omitted.

Both of these methods of fusing or brazing are old and well known in the art. Although they are desirable procedures for some types of welding, it has been found that these operations are too time consuming and costly when used to attach contacts to the circuit breaker contact arms. In addition, these methods usually result in undesirable annealing of the supporting arm. As a result, some manufacturing concerns have replaced the above method and now attach contacts by means of spot welding techniques. These techniques call for a short welding period in which the welding currents are greatly in excess of those used in the resistance brazing methods. Hence, in order to use the more desirable spot welding technique, it has been necessary to adapt the contact for this particular type of attachment process.

One such adaption is to provide the contacts with a ser rated back. The serrations, of cavity or protrusion type, permit local areas of high heat at the juncture of the contact and its support. Hence, with these restricted areas of engagement, the cross-sectional area carrying the welding current will have a high concentration of current and is thus much better adapted for spot welding operations than are the fiat back contacts which tend to distribute the heat throughout.

These serrations may take any one of several forms. Among the more common, depending upon the shape of the contact, is that of a concentric circular ring so made that a cross-sectional of the contact reveals alternate hills and vales concentric with the center of the contact. Another form is that which includes truncated pyramids on the back of the contact. These truncated pyramids may take the form of either protrusions or cavities. In any event, they may be pressed into the contact as it is molded or they maybe pressed into the contact following the sintering and infiltration step. After the serrations have been placed on the back of the contact, a pure silver coating is placed thereon.

The amount of silver on the back of the contact may be so controlled as not to obliterate the serrations thereon or it may be deliberately large so that the silver can be coined into serrations after the contact is finished.

Still another device which has been used is to put a coating of silver solder on the flat back of a contact. This is either left flat or struck with a tool so as to form the solder itself into a serrated pattern.

Extensive experimentation has resulted in the conclusion that none of the aforementioned methods give the positive weldability which I am able to obtain with my invention.

My invention may be used on contacts which have either a flat or serrated back. To this backing, I attach a coating of silver solder in such a manner that the silver solder is placed in a puddle or globule covering the entire back and allowed to assume its natural radius.

Since the natural radius is highly curved, when the correct amount of solder is used, a point contact is made between the surface of the solder and the surface of the copper supporting arm when the contact is placed thereon. This has the very decided advantage of concentrating in a relatively small area such as a point contact, the welding current. This obviously provides a quick adequate heating of the solder so that it can alloy with the support material which is usually copper.

Although many different types of solder can be used with the method of my invention, one in particular has been found to give superior results. This solder has been known as Sil-Fos which is silver brazing alloy having a composition of 80% copper, 15% silver and 5% phosphorus.

Further experimentation on this type of contact has resulted in the conclusion that it is highly advisable to have all of the corners and edges of the contact rounded so that an electrical are drawn between the contacts when they are separated will not stay and hang on to the sharp edges thereof.

Accordingly, it is a practice to tumble such contacts in an abrasive medium which will round all of the sharp edges and corners thereof. However, when the contacts made in accordance with my invention have been tumbled by this process, it has been found that the contact and solder are burnished and polished so that both have the same color and lustre. Hence, when these contacts are removed from the tumble step, it becomes very diflicult, if not impossible in many cases, to distinguish the solder sides of the contact from the contact surface. This is particularly true of contacts which have been molded with a face radius.

Therefore, I have provided a method to circumvent this disadvantage resulting from the tumbling step. After the contacts have had their corners and edges rounded, I propose to place these contacts in a furnace and heat them in an oxidizing atmosphere such as air. The tem perature in the electrical furnace is raised to a sufiicient degree to oxidize the copper in the solder and thereby discolor the solder while not changing the appearance of the contact.

Thus, the two surfaces which were previously burnished and polished to the same color and lustre are now readily distinguishable.

I have found that if the contacts are exposed to a temperature between 250 and 450 C. for a period of approximately one half hour, the copper in the solder is oxidized and discolored sufficiently so as to render it distinguishable from the remaining surfaces of the contact.

Hence, with my invention, I am able to produce a contact which has the added advantage of positive weldability and one which has its surfaces readily distinguishable. Such a contact results in more economical assembly since it is easily and quickly readied for positioning in a welding setup where the spot welding type technique is faster than the conventional brazing methods.

I claim:

1. An electrical contact of refractory material from the group consisting of molybdenum, tungsten and carbides thereof fusible to an electrically conductive metallic supporting arm, said contact having all of its corners and edges rounded, and on the back thereof, a globule of solder consisting essentially of percent copper, 15 percent silver and 5 percent phosphorus which is curved, said solder surface to provide a point contact between the surface of the solder and the supporting arm, and said solder surface being oxidized so that the copper therein is oxidized and discolored, thereby making the color of the solder surface distinguishable from that of the uncoated face of the contact.

2. An electrical contact of refractory material from the group consisting of molybdenum, tungsten and carbides thereof, for fusing to an electrically conductive metallic supporting arm, said contact having all of its corners and edges rounded, and a serrated back, and on the serrated back thereof, a surface of solder consisting essentially of 80 percent copper, 15 percent silver and 5 percent phosphorus which is curved, said solder surface to provide a point contact between the surface of the solder and the supporting arm, and said solder surface further being oxidized so that the copper therein is oxidized and discolored, thereby making the color of the solder surface distinguishable from that of the uncoated face of the contact.

References Cited in the file of this patent UNITED STATES PATENTS 1,658,713 Fuller Feb. 7, 1928 2,096,924 Schwarzkopf Oct. 26, 1937 2,179,753 Mebold Nov. 14, 1939 2,192,751 Melchior Mar. 5, 1940 2,196,304 Hensel Apr. 9, 1940 2,216,510 Burns, Oct. 1, 1940 2,234,834 Scott Mar. 11, 1941 2,285,293 Mansfield June 2, 1942 2,319,240 Larsen May 18, 1943 2,319,610 Lake May 18, 1943 2,419,469 Spiro Apr. 22, 1947 2,429,222 Ehrhardt Oct. 21, 1947 2,504,906 Tremblay Apr. 18, 1950 2,575,808 Halverson Nov. 20, 1951 

1. AN ELECTRICAL CONTACT OF REFRACTORY MATERIAL FROM THE GROUP CONSISTING OF MOLYBDENUM, TUNGSTEN AND CAR BIDES THEREOF FUSIBLE TO AN ELECTRICALLY CONDUCTIVE METAL-LIC SUPPORTING ARM, SAID CONTACT HAVING ALL OF ITS CORNERS AND EDGES ROUNDED, AND ON THE BACK THEREOF, A GLOBULE OF SOLDER CONSISTING ESSENTIALLY OF 80 PERCENT COPPER, 15 PERCENT SILVER AND 5 PERCENT PHOSPHORUS WHICH IS CURVED SAID SOLDER SURFACE TO PROVIDE A POINT CONTACT BETWEEN THE SURFACE OF THE SOLDER AND THE SUPPORTING ARM, AND SAID SOLDER SURFACE BEING OXIDIZED SO THAT THE COPPER THEREIN IS OXIDIZED AND DISCOLORED, THEREBY MAKING THE COLOR OF THE SOLDER SURFACE DISTINGUISHABLE FROM THAT OF THE UNCOATED FACE OF THE CONTACT. 